The invention relates to a circuit arrangement and electronic control gear for starting and operating discharge lamps. Electronic control gear for gas discharge lamps are used increasingly as a result of their advantages in comparison with the conventional ballasts, such as increased light quality, improved luminous efficiency and automatic disconnection of the gas discharge lamps at the end of life. Until now, circuits with a full-bridge which operate the lamp with a type of alternating direct current have primarily been used for high-pressure gas discharge lamps. This is necessary since most high-pressure gas discharge lamps cannot be operated on alternating currents with a higher frequency owing to resonances in the burner vessel. In this case, two principle configurations are distinguished from one another. Firstly, the configuration with a step-down converter with a downstream full bridge, and secondly a full bridge with an integrated step-down converter functionality. For reasons of costs, the latter option has been increasingly used for some time. For starting, a pulse starting device is generally used, for which a further switch for triggering the starting pulse is required. In the case of lamp types which have a comparatively low starting voltage, resonant starting methods are also used.
In this case, a half-bridge branch of the full-bridge with a starting inductor and a starting capacitor with a small capacitance is used for the resonant starting. For the step-down converter stage, which produces the low-frequency square-wave current after starting of the lamp, the second half-bridge branch with the step-down converter inductor and the step-down converter filter capacitor (greater capacitance) is used. The step-down converter stage in this case functions in the favorable intermittent operating mode, which makes zero voltage switching possible.
US2004/183463A1 has disclosed a circuit which operates a gas discharge lamp with a full-bridge, the full-bridge being split into two half-bridge branches which function differently and of which one is operated at a low frequency, whereas the other can also be operated at a high frequency. An LCR resonant circuit, which is excited by the half-bridge branch functioning at a high frequency, is provided for the starting of the gas discharge lamp.
All of these arrangements are very cost-intensive, and therefore in recent times attempts have increasingly been made to replace the cost-intensive full-bridge with a more favorable half-bridge. Said half-bridge uses a small capacitor for operation. The half-bridge in this case runs in the non-intermittent operating mode during step-down operation. In the non-intermittent operating mode, the value of the capacitor capacitance is of little importance for the filtering of the lamp current. A small capacitance which acts as a starting capacitor for the resonant starting is therefore used in this case. The non-intermittent operating mode has the disadvantage, however, that considerably higher switching losses occur and it results in unfavorable dimensioning of the step-down converter inductor (a high inductance is required for the step-down converter inductor, which results in a large physical shape and more losses). A large inductor also requires more space and results in increased costs.